The present invention generally relates to casting equipment and processes. More particularly, the invention relates to reducing surface defects in directionally-solidified castings, including single-crystal (SX) and directionally-solidified (DS) castings.
Hot gas path components of gas turbines, such as blades (buckets), vanes (nozzles) and combustor components, are typically formed of nickel-, cobalt- or iron-based superalloys characterized by desirable mechanical properties at turbine operating temperatures. Because the efficiency of a gas turbine is dependent on its operating temperatures, there is a demand for hot gas path components that are capable of withstanding higher temperatures. As the material requirements for gas turbine components have increased, various processing methods and alloying constituents have been used to enhance the mechanical, physical and environmental properties of components formed from superalloys. For example, buckets, nozzles and other components employed in more demanding applications are often cast by directional casting techniques to have DS or SX microstructures, characterized by a crystal orientation or growth direction in a selected direction to produce columnar polycrystalline or single-crystal articles. As known in the art, directional casting techniques generally entail pouring a melt of the desired alloy into an investment mold held at a temperature above the liquidus temperature of the alloy, and then gradually withdrawing the mold into a cooling zone where solidification initiates at the base of the mold and the solidification front progresses upward.
Investment molds are typically formed by dipping a wax or plastic model or pattern of the desired component into a slurry comprising a binder and a refractory particulate material to form a slurry layer on the pattern. Common materials for the refractory particulate material include alumina, silica, zircon and zirconia, and common materials for the binder include silica-based materials, for example, colloidal silica. A stucco coating of a coarser refractory particulate material is typically applied to the surface of the slurry layer, after which the slurry/stucco coating is dried. The preceding steps may be repeated any number of times to form a shell mold of suitable thickness around the wax pattern. The wax pattern can then be eliminated from the mold, such as by heating, after which the mold is fired to sinter the refractory particulate materials and achieve a suitable strength. To produce hollow components, such as turbine blades and vanes having intricate air-cooling channels, one or more cores are provided within the shell mold to define the cooling channels and any other required internal features. Cores are typically prepared by baking or firing a plasticized ceramic mixture, and then positioned within a pattern die cavity into which a wax, plastic or other suitably low-melting material is introduced to form the pattern for the mold. Once solidified, the pattern with its internal cores can be used to form the shell mold as described above.
A particular known investment casting process employs a Bridgman-type furnace to create a heated zone surrounding the mold, and a chill plate at the base of the mold. Solidification of the molten alloy within the mold occurs by gradually withdrawing the mold from the heated zone and into a cooling zone beneath the heated zone, where cooling occurs by convection and/or radiation. A high thermal gradient is required at the solidification front to prevent nucleation of new grains during directional solidification processes. For example, commonly-assigned U.S. Pat. No. 6,217,286 to Huang et al. discloses a casting process that achieves a high thermal gradient at the solidification front with the use of a cooling zone that comprises a tank containing a liquid cooling bath, such as molten tin or another molten metal.
Mechanical properties of DS and SX articles depend in part on the avoidance of casting defects, including pitting and other surface defects that may result from chemical reactions with the mold during the solidification process. One potential source of surface defects is a molten metal coolant noted above for achieving high thermal gradients during solidification. An undesirable cast surface reaction may occur if the coolant penetrates the mold by infiltration of porosity or a crack in the mold prior to the completion of the casting operation. Consequently, shell molds used in investment casting processes must exhibit sufficient strength and integrity to survive the casting process.
Additional challenges are encountered when attempting to form castings of alloys that contain an appreciable amount of one or more reactive materials, including nickel-based superalloys that contain niobium, titanium, zirconium, yttrium, tantalum, tungsten, rhenium and potentially other elements that tend to readily react with oxygen when molten or at an elevated temperature. For this reason, surfaces of molds and cores used in the casting of materials containing reactive elements may be provided with protective barriers known as facecoats. Facecoats are generally applied to mold and core surfaces in the form of a slurry, which may be dried and sintered prior to the casting operation or undergo sintering during the casting operation. Typical facecoat slurries comprise a refractory particulate material in an aqueous-based inorganic binder, optionally with various additional constituents such as organic binders, surfactants, dispersants, pH adjusters, etc., to promote the processing, handling, and flow characteristics of the slurry. The refractory particulate material is chosen on the basis of being sufficiently unreactive or inert to the particular reactive material being cast. Various facecoat materials have been used and proposed, including those containing yttria (Y2O3), alumina (Al2O3), and zirconia (ZrO2) in a colloidal silica binder.